Randy V

Lately, I've been noticing that the oil pressure reading on the dash gauge is around 4 at idle, and pegged at any RPM above idle (the gauge has worked properly before and still appears to be accurate). Additionally, I'm having a problem with one of the #3 cylinder's hydraulic lifters overpressurizing at higher RPM's causing a miss (verified on dyno).

What device/system controls oil pressure on my '79 auto? Is there some part that may have failed, resulting in elevated oil pressure?

Thanks for your help.

Incendier

Randy:

Restriction or failed pressure relief are my guesses.

First, changed the oil filter recently? Sounds stupid, I know...

I'm inclined to go with a failed pressure relief valve. It's north of the oil filter, screwed into the block. It's the left-hand of the two big bores, the smaller-size bolt head (the right is the bypass valve). If that valve has failed, it could raise - or, rather, fail to limit, the oil pressure as you describe. Should be a simple, if messy, matter to remove it, clean up the spring and piston assembly, and replace.

Aside from that, if something random has clogged or otherwise restricted an oil gallery, pressure would increase through that route and overall sytem pressure should rise, although a properly functioning relief valve should keep it in check.

Good luck. Buy some shop towels.

Randy V

Thanks James, sounds like that may be the culprit.

Yes, I changed the oil and filter 10 days ago, after running for 30 minutes with an engine flush additive, resulting in little or no improvement.

Steve Cattaneo

Randy,
Assuming the gauge is accurate, best to verify with a gauge. The pressure relief valve control the pressure my guess is a stuck valve. Take the bolt out then the spring, pull and clean the valve. Page 20 of your 928 book.

Randy V

Good advice - thanks Steverino!

Steve Cattaneo

You guys type too fast.

Steve Cattaneo

Randy,
What happened to the smilies?

Randy V

Steveo,

Click on the 'Post Reply' link at the bottom of the thread instead of using the 'QuickReply' box. That's where the smileys live. <img border="0" alt="[byebye]" title="" src="graemlins/wave.gif" />

Jim bailey - 928 International

the oil pressure relief blows off at 9 Bar ....if you have four bar on the guage at idle it is below the threshold of what the bypass controls . I would suspect a incorrect reading sending unit , guage or wiring . There is a differential pressure relief (early cars with oil cooler) which opens if there is a 1/2 bar differential in pressure of oil flowing through the cooler and that coming from the pump . If the oil thermostat were stuck and the bypass stuck possibly this would cause a high reading but I tend to think your car is not being truthful and is telling stories .

Randy V

Thanks Jim, but I think the gauge is OK as pressure readings tend to decrease as the engine heats up.

Add to that the fact that the hydraulic lifter on #3 cylinder is balky at high RPM's resulting in a miss, all indications point to high oil pressure.

My '79 does not have the oil cooling loop.

Any other thoughts?

Jim bailey - 928 International

The miss could be from a broken valve spring which could be overpowered by oil pressure otherwise it really should not be happening , the minimum oil pressure warm at 4,000 rpm is 5 Bar 72 psi . You really should confirm the pressure with a separate gauge .

Randy V

I had suspected a broken valve spring before the high oil pressure. It's looking like it may be a combination of both. I'll test pressure with a separate gauge as suggested.

Thanks JB.

Erik - Denmark

Randy,
The oil pressure at idle is highly depending of the viscosity/temperature.

Are you sure you used same viscosity oil and the engine is hot (not only the water but also the oil) when you read the 4 Bar.
I think even in California you have winter season now - I know not winter like Scandinavia, but still lower temperature than normal

I just went for shopping - drove 25 km (15 miles) - shopping 45 min. - when I started the car it shows 4 Bar - when arriving home (another 25 km) it showed 3 Bar.

If your engine is healthy with good bearings, I don't think you have a problem.

Ed Ruiz

Randy:
When I first bought my 86.5S, it had rather noisy lifters. When it came time for the first oil change, I decided to pamper the engine with 9 quarts of Mobil 1 15W50. Within a month, the lifter noise was gone. During the next oil change, I noticed the old oil was as black as night, and a bit sludgy. Since that oil change, all subsequent oil changes resulted with old oil that came out brown but translucent, and without sludge. Also, the reading on the oil pressure gauge got a bit higher and stayed more consistent.

I sold the 86.5S several years ago, but still use Mobil 1 in all my cars. It costs more, but I think they're worth it! YMMV.

Shane

OK,

I use Mobil 1 in all my vehicals but my 928's. At least until I get the engines out and regasket them.